The sludge resulted from any water processing for, for example, water supply and sewerage, industrial wastewater, human waste and the like is subjected to a thermal disposal within an incinerator. The sludge is preferably incinerated after a concentration processing for reducing a moisture content of the sludge. A known sludge concentration apparatus (including a case of using dehydration) includes a centrifugal separator which is also referred to as a decanter (for example, see Patent Literatures 1 and 2).
FIG. 12 shows a schematic configuration of a horizontal decanter (for example, see Patent Literatures 3, 4 and 5). As shown in FIG. 12, the horizontal decanter, which is generally designated as 100, is configured so that a bowl 101 rotatable about a horizontal shaft and a screw conveyor 102 disposed in the bowl 101 so as to coaxially rotate about the horizontal shaft of the bowl 101 are housed within a casing 103.
The bowl 101 has a conical-shaped portion at one end, and applies a centrifugal force to the sludge which is a subject to be processed. The conical-shaped portion acts as a beach portion on which the concentrated sludge conveyed by the screw conveyor 102 is separated from a liquid pool. The exhaust port 104 for the concentrated sludge is formed on a tip of the conical-shaped portion. A trunk of the bowl 101 acts as the liquid pool (pool portion) for the being processed sludge. The exhaust port 105 for the separated liquid is formed on the opposed end of the exhaust port 104. The spiral screw vane 102a and an outlet. 102b for feeding the sludge (feed sludge) into the bowl 101 are formed on the trunk of the screw conveyor 102.
In such configuration, when the sludge (feed sludge) is fed continuously into the rotating bowl 101, in the pool portion of the bowl 101, the solid contained in the sludge settles to a peripheral wall surface of the bowl 101 due to the centrifugal force. Accordingly, the screw conveyor 102 rotates at a differential speed relative to a rotation speed of the bowl 101 via a gear box 106, thereby conveying the concentrated sludge within the bowl 101 forward the beach portion. At the beach portion, the concentrated sludge is separated from the liquid pool and exhausted from the exhaust port 104 for the concentrated sludge. On the other hand, the separated liquid is overflown and exhausted from the exhaust port 105.
The bowl 101 is rotated by a main drive motor 107. The main drive motor 107 controls a rotation speed (N) of the bowl 101 though an inverter control. The screw conveyor 102 is configured so that the gear box 106 and a drive motor (back drive motor) 108 control the rotation speed to rotate as the differential speed (ΔN) relative to the rotation speed of the bowl 101.
In general, the decanter 100 performs a variable control on a convey torque of the screw conveyor 102 and/or the differential speed (ΔN) so that the moisture content of the concentrated sludge satisfies a predetermined target value. However, if the energy saving, the reduction of CO2 emissions or the like should be promoted to realize more effective operation of the entire sludge processing system, it is not preferable to control the decanter 100 only by a single control loop. It is desired to perform the optimal operation of the decanter 100 in accordance with the operating condition of the incinerator, but there is no fact that such a control means has been implemented.